1. Field
Embodiments of the invention relate to the field of digital imaging; and more specifically, to subject illumination for capturing images.
2. Background
Cameras capture images by recording the light reflected from a subject. It is necessary for the subject to be adequately illuminated so that a sufficient amount of light is reflected into the camera for recording.
A camera may use an electronic image sensor to record images. Electronic image sensors may produce an “image” even when no light falls on the sensor. This “image” represents noise produced by the sensor. There may be other forms of noise produced by the sensor as well. It is desirable that the image produced by the subject produce signals that are substantially greater than the noise signals produced by the sensor.
Increasing the amount of light reflected onto the image sensor by increasing the amount of light illuminating the subject may improve the ratio of signal produced by the subject image to signal produced as noise, the signal to noise ratio (SNR), thus improving the quality of the captured image.
Still cameras may employ flash lighting where an extremely bright light of short duration illuminates the subject at the moment the subject image is captured. Cameras that capture a rapid succession of images to provide a moving picture, such as video cameras, may provide a light that provides a steady bright light to illuminate the subject continuously while images are being captured. These auxiliary light sources increase the amount of light falling on and reflected by the subject to improve the quality of images captured.
The amount of light falling on a subject may be termed illuminance, which is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Illuminance and luminous emittance may be measured in lux. Thus, an auxiliary light source provides a level of luminous emittance to create a level of illuminance on a subject. A camera will require a minimum level of illuminance of the subject to produce an image of acceptable quality. If the subject illuminance is low, the SNR will be low and the image will appear grainy or snowy. It is desirable to provide a subject illuminance that results in a SNR that is above the level necessary to provide an image of acceptable quality.
Cameras with digital image sensors are increasingly being added to various mobile devices, such as mobile telephones, personal digital assistants (PDA), mobile computers, and the like. When a camera is added to a mobile device that provides functions in addition to capturing images, the image capture may be a secondary function. As such, it is desirable to minimize the cost and space required to provide the image capture function. It would be desirable to provide subject illumination to improve the quality of captured images without adding a dedicated light source for image capture to the mobile device.